1. Field of the Invention
The present invention relates to signal processing, and, in particular, to the routing of signals for synchronous and/or asynchronous communications.
2. Description of the Related Art
In a communication system, different nodes communicate with one another by exchanging signals that conform to a specified transmission protocol known by the nodes. The transmission protocol enables the nodes to decode information or data contained in the signals. Communication systems typically support one of either synchronous or asynchronous communications. In synchronous communications, signals are transmitted in a continuous steady stream in which the signals are received in the same temporal sequence in which they were transmitted. In asynchronous communications, signals are transmitted in discrete, non-steady packets of data that may possibly be received out of order--that is, in a temporal sequence different from that in which the packets were transmitted.
Asynchronous Transfer Mode (ATM) is a particular standard for asynchronous communications. According to the ATM standard, signals are transmitted in ATM packets called cells that contain 53 bytes of data, where, in each cell, the first 5 bytes are reserved for overhead data and the remaining 48 bytes are available for payload data. Overhead data correspond to information used by the nodes and other network components to route the cells and/or reconstruct the streams of communication signals. For example, overhead data may include a source code, a destination code, an error correction code, and other data depending on the type of interface. Payload data correspond to the streams of communication signals being exchanged between the network nodes. For example, in a telephony system, payload data may correspond to the telephone audio signals transmitted from one node to another.
SONET (Synchronous Optical NETwork) is a standard for synchronous communications over optical fiber networks. The SONET standard defines a number of different formats for encoding data for exchange between network nodes. One of these formats is the STS-1 frame in which data are encoded into a matrix consisting of byte-size elements arranged in 9 rows and 90 columns, in which the first 3 columns are reserved for overhead data and the remaining 87 columns are used for both overhead and payload data. Each STS-1 frame is transmitted in 125 microseconds at a fiber data rate referred to as the OC-1 data rate.
Another SONET format is the STS-3 frame in which data are encoded into a matrix consisting of 9 rows and 270 columns, in which the first 9 columns are reserved for overhead data and the remaining 261 columns are used for both overhead and payload data. Each STS-3 frame is transmitted in 125 microseconds at the OC-3 fiber data rate.
Yet another SONET format is the STS-12 frame in which data are encoded into a matrix consisting of 9 rows and 1080 columns, in which the first 36 columns are reserved for overhead data and the remaining 1044 columns are used for both overhead and payload data. Each STS-12 frame is transmitted in 125 microseconds at the OC-12 fiber data rate.
Still another SONET format is the STS-48 frame in which data are encoded into a matrix consisting of 9 rows and 4320 columns, in which the first 144 columns are reserved for overhead data and the remaining 4176 columns are used for both overhead and payload data. Each STS48 frame is transmitted in 125 microseconds at the OC-48 fiber data rate.
The selected data rate dictates which SONET frame format is used. In any case, the data encoded in any of the SONET formats are transmitted sequentially within each row from left to right and row by row from top to bottom.
FIG. 1 shows a single path 100 linking two end nodes 102 (e.g., two telephones) in a conventional communication network, for purposes of demonstrating different types of overhead data. At the center of path 100, is a switch 106 that handles the routing of signals between end nodes 102. In addition, path 100 has zero, one, or more repeaters 104 between switch 106 and each end node 102 that act as amplifiers for the transmitted signals.
Signals transmitted over a communication network having paths like path 100 of FIG. 1 may contain three different types of overhead data: path data, line data, and section data. Path data refers to overhead data communicating information between end nodes 102 of a communication network. Line data refers to overhead data communicating information between an end node 102 and a switch 106 internal to the communication network. Section data refers to overhead data communicating information between an end node 102 and a repeater 104 internal to the communication network, between two repeaters 104, or between a repeater 104 and a switch 106. Overhead data in each of the different SONET formats includes path data, line data, and section data.
It is often desirable to connect two different types of communication networks together to enable nodes of each network to communicate with nodes of the other. In this case, data formatted for communication over one network must be re-packaged for communication over the other network. The SONET and ATM standards define how to package ATM data for transmission over a SONET synchronous network. In particular, the 53-byte ATM cells are packed into the payload portion of the SONET frames. Such transmissions are referred to as ATM-over-SONET.
For example, for the OC-1 data rate, the 53-byte ATM cells are packed into 86 of the 87 payload columns of each of the 9 rows of each STS-1 frame. Since there are 774 payload bytes in each STS-1 frame, the first STS-1 frame can hold 14 complete ATM cells and the first 32 bytes of a 15.sup.th ATM cell. The remaining 21 bytes of the 15.sup.th ATM cell are packed into the initial payload fields of the second STS-1 frame, followed by another 14 complete ATM cells and the first 11 bytes of the 301 ATM cell; and so on. In this way, the ATM cells are efficiently packed into the payload portions of the STS-1 frames.
Similarly, for the STS-3, STS-12, and STS-48 frames, ATM cells are packed into the payload portions with fractions of ATM cells overlapping consecutive frames, as necessary. Because SONET is a synchronous system that requires a steady stream of signals, if cells are not received in time from the ATM network to fill payload fields of the current SONET frame, idle cells containing null data are used to maintain the synchronous flow of signals. For a SONET system with multiple ATM inputs, the different inputs are polled when filling the SONET frames, with each input having a unique identifier so that the SONET system can keep track of which ATM input supplied each ATM cell. Of the ATM cells arrive at a sufficiently high rate, the SONET system may have difficulty keeping up with the ATM data flow.